UU

Uu, often capitalized as UU, refers to UUencode, a historical binary-to-text encoding used to transfer binary data over channels that supported only 7-bit ASCII, such as email and Usenet posts. The method arose with the Unix-to-Unix Copy Program (UUCP) in the 1980s and remained common through the 1990s. Today it is largely superseded by MIME and base64 but persists in some legacy systems and archives.

UUencode works by converting binary data into printable ASCII characters. Data is processed in blocks of three bytes, which are mapped into four 6-bit values. Each 6-bit value is then increased by 32 to fall within the printable ASCII range, with the backtick character used to represent a zero value in the 6-bit groups. Each line of output begins with a single character that encodes the number of input bytes on that line (the line length plus 32). A typical encoded file is framed by a header line such as "begin 644 filename" and ends with a line containing "end." Decoding is performed by a corresponding uudecode tool, which restores the original filename and data.

Compared with modern encodings, UUencode is less space-efficient and less flexible, but it was well suited to the constraints of early email and Usenet workflows. It remains of historical interest for understanding early internet file transfer methods and for decoding older messages and archives that used the format.